Pressure wave generator

Abstract

A pressure wave generator comprising a housing, a multiple diaphragm having a first diaphragm and a second diaphragm, a gap is provided between the first diaphragm and the second diaphragm, the first diaphragm is spaced at least 5 mm from the second diaphragm to form the gap, and the gap comprise an interspace fluid being a gas under pressure or a liquid. The first diaphragm and the second diaphragm each has an inner attachment region indirectly or directly connected to a moving assembly and an outer edge indirectly or directly connected to the housing. The pressure wave generator also has a leak detection port in fluid communication with the gap, and a drive system for driving the moving assembly. The first diaphragm and the second diaphragm is moveable in a reciprocating motion within the housing to generate the pressure waves as the moving assembly moves.

Description

[0001] PRESSURE WAVE GENERATOR

[0002] FIELD OF THE INVENTION

[0003] The present invention relates to a pressure wave generator with a multiple diaphragm. More particularly, but not exclusively, it relates a pressure wave generator with a multiple diaphragm for a cryogenic system.

[0004] BACKGROUND OF THE INVENTION

[0005] Cryogenic systems are used for producing, maintaining and storing cryogenic liquids or cryogens, which are liquefied gases, and other substances at very low temperatures. Cryogens are produced by liquefying gases by cooling them until they change state to liquid. Cryogenic systems are now widely used in a number of different industries as they have proven to be useful in many different processes.

[0006] To help provide cooling in cryogenic systems, pressure wave generators may be used. In these systems the pressure wave generator generates pressure waves that cause compression and expansion of gas that in turn causes the temperature of the gas to increase or decrease. The cryogenic system make use of the gas temperature decreases, to provide a cooling effect. Examples of cryogenic systems being driven by pressure wave generators include pulse tube cryocoolers, and Stirling cryocoolers.

[0007] Some pressure wave generators use reciprocating diaphragms to generate pressure waves. These diaphragms in a pressure wave generator are typically subject to high stresses. There may therefore be a need for improving pressure wave generators to provide, among other things, an improved lifespan of these types of pressure wave generators. In cryogenic systems, this may be particularly advantageous, where hazardous gases are being cooled and / or used in the system.

[0008] In this specification, where reference has been made to external sources of information, including patent specifications and other documents, this is generally for the purpose of providing a context for discussing the features of the present invention. Unless stated otherwise, reference to such sources of information is not to be construed, in any jurisdiction, as an admission that such sources of information are prior art or form part of the common general knowledge in the art. For the purpose of this specification, where method steps are described in sequence, the sequence does not necessarily mean that the steps are to be chronologically ordered in that sequence, unless there is no other logical manner of interpreting the sequence.

[0009] It is an object of the present invention to provide an improved pressure wave generator with a multiple diaphragm which overcomes or at least partially ameliorates some of the abovementioned disadvantages, or which at least provides the public with a useful choice.

[0010] BRIEF DESCRIPTION OF THE INVENTION

[0011] In accordance with a first aspect of the present invention, there is provided a pressure wave generator comprising: a housing comprising an inlet and an outlet for inflowing and outflowing pressure waves generated within the pressure wave generator; a multiple diaphragm comprising: a first diaphragm; and a second diaphragm; the first diaphragm and the second diaphragm each having an inner attachment region indirectly or directly connected to a moving assembly and an outer edge indirectly or directly connected to the housing; a gap provided between the first diaphragm and the second diaphragm, the first diaphragm is spaced at least 5mm from the second diaphragm to form the gap, and the gap comprise an interspace fluid being a gas under pressure or a liquid and; a leak detection port in fluid communication with the gap; and a drive system for driving the moving assembly, the first diaphragm and the second diaphragm moveable in a reciprocating motion within the housing to generate the pressure waves as the moving assembly moves.

[0012] In some configurations, the first diaphragm is operatively located adjacent to the second diaphragm such that the first diaphragm and the second diaphragm are configured to move together upon movement of the moving assembly. In some configurations the multiple diaphragm has two diaphragms consisting of the first diaphragm and the second diaphragm.

[0013] In some alternative configurations, the multiple diaphragm has more than two diaphragms comprising the first diaphragm, the second diagram, and one or more additional diaphragms.

[0014] In some configurations, the gap is one of a vacuum, liquid-filled, or gas-filled.

[0015] In some configurations, the first diaphragm is spaced 5mm to 25mm from the second diaphragm.

[0016] In some configurations, the first diaphragm and second diaphragm are connected to the housing and moving assembly by fastening components, adhesives, welding, brazing and / or clamped in place by the pressure created within the pressure wave generator.

[0017] In some configurations, the first and second the diaphragms comprise outer edges connected to a housing spacer.

[0018] In some configurations, the first and second the diaphragms comprise inner attachment regions connected to an inner spacer.

[0019] In some configurations, a compression space is formed within the housing and is formed by the multiple diaphragm and the end plate of the housing.

[0020] In some configurations, the interspace fluid in the gap is the same as a working fluid provided in the compression space.

[0021] In some configurations, the interspace fluid has an interspace pressure being lower than the lowest pressure of the working fluid in the compression space.

[0022] In some configurations, the interspace pressure in the gap is 50% to <100% of the pressure of the working fluid in the compressor space.

[0023] In some configurations, the interspace pressure in the gap is 10 bar to 20 bar and the pressure of the working fluid in the compressor space is 25 bar plus or minus 5 bar.

[0024] In some configurations, a connection is provided between the compression space and a gas spring space located between the housing and an end plate, to ensure the pressures in the compression space and gas spring space are approximately the same.

[0025] In some configurations, a feed is provided from the gas spring space to the gap to provide the interspace fluid. In some configurations, a pressure regulator is connected to the gap to introduce and / or maintain interspace pressure.

[0026] In some configurations, a sensor or other component is connected to the leak detection port to detect or respond to changes in the vacuum or pressure of the gap.

[0027] In some configurations, the component is a vacuum switch, the vacuum switch being fluidly connected to the leak detection port.

[0028] In some configurations, the vacuum switch comprises a first switch position and a second switch position, where the switch is configured to trigger a shut-down of a system if a leak is detected.

[0029] In some configurations, the gap between the first diaphragm and the second diaphragm comprise a gas under pressure.

[0030] In some configurations, the gap is filled with an inert gas, optionally the inert gas is helium.

[0031] In some configurations, the gap between the first diaphragm and the second diaphragm comprises a liquid.

[0032] In some configurations, liquid in the gap is a coolant.

[0033] In some configurations, the pressure wave generator comprises a pump to pump the coolant through the gap.

[0034] In some configurations, the gap comprises a pressure being greater than each of the compression space and a housing chamber.

[0035] In some configurations, the pressure wave generator comprises a pressure sensor fluidly connected to the gap.

[0036] In some configurations, the pressure sensor detects the magnitude of pressure change in the gap.

[0037] In some configurations, the pressure sensor detects the rate of pressure change in the gap.

[0038] In some configurations, the drive system is configured to operate the moving assembly such that the diaphragms are actuated at a frequency of approximately 30Hz to 50Hz.

[0039] In some configurations, the diaphragms are configured to travel approximately

[0040] 5mm due to the reciprocating motion. In some configurations, the diaphragms are 500mm or 800mm in diameter.

[0041] In some configurations, the second diaphragm comprises a weakened region or a weakened structure such that the second diaphragm is configured to fail before the first diaphragm.

[0042] In some configurations, the pressure wave generator comprises a pair of multiple diaphragms, the pair of multiple diaphragms comprising a first multiple diaphragm and a second multiple diaphragm and wherein each of the first and second multiple diaphragms are multiple diaphragms as outlined in relation to the first aspect above.

[0043] In accordance with a second aspect of the present invention, there is provided a method of detecting a leak in a multiple diaphragm pressure wave generator comprising: providing a pressure wave generator comprising a multiple diaphragm as outlined in relation to the first aspect above; detecting a parameter in the gap of the multiple diaphragm; wherein a change in the parameter provides an indication of a leak of the multiple diaphragm.

[0044] In some configurations, detecting a change in pressure in the gap below or above a threshold is an indication of a leak.

[0045] In some configurations, detecting a change in pressure below a threshold is an indication of a leak.

[0046] In some configurations, a pressure sensor is provided to detect the change in the parameter.

[0047] In some configurations, a vacuum switch is provided to detect the parameter change.

[0048] In some configurations, the method further comprises detecting the rate of pressure change in the gap.

[0049] In some configurations, a first rate of pressure change indicates a failure of the first diaphragm, and a second rate of pressure change indicates a failure of the second diaphragm.

[0050] In some configurations, the method further comprises triggering a shut down protocol when a leak has been detected. In accordance with a third aspect of the present invention, there is provided a cryogenic system comprising: the pressure wave generator as outlined in relation to the first aspect above; and a cryogenic refrigerator operatively connected to the pressure wave generator such that the cryogenic refrigerator is driven by the pressure waves generated by the pressure wave generator.

[0051] In some configurations, the cryogenic refrigerator comprises a compressor, a heat exchanger, a regenerator, and a cold end.

[0052] In some configurations, the cryogenic system is a pulse tube cryocooler.

[0053] In some configurations, the cryogenic system is a Stirling cryocooler.

[0054] Other aspects of the invention may become apparent from the following description which is given by way of example only and with reference to the accompanying drawings.

[0055] As used herein the term "and / or" means "and" or "or", or both.

[0056] As used herein "(s)" following a noun means the plural and / or singular forms of the noun.

[0057] The term "comprising" as used in this specification and claims means "consisting at least in part of". When interpreting statements in this specification and claims which include that term, the features, prefaced by that term in each statement, all need to be present but other features can also be present. Related terms such as "comprise" and "comprised" are to be interpreted in the same manner.

[0058] BRIEF DESCRIPTION OF THE DRAWINGS

[0059] The invention will now be described by way of example only and with reference to the drawings in which:

[0060] Figure 1 shows a schematic diagram of a pressure wave generator with a multiple diaphragm. Figure 2 shows another configuration of a pressure wave generator with a multiple diaphragm.

[0061] Figure 3 shows the pressure wave generator of figure 2 with a downwardly displaced moving assembly.

[0062] Figure 4 shows a partial schematic view of the pressure wave generator of figure 2.

[0063] Figure 5 shows a detailed schematic view of the pressure wave generator with spacers.

[0064] Figure 6 shows a detailed schematic view of another pressure wave generator with multiple diaphragms attached directly to the housing.

[0065] Figure 7 shows a schematic of a pulse tube cryocooler connected to the pressure wave generator.

[0066] Figure 8 shows an exploded view of the components of the pressure wave generator shown in figure 2.

[0067] Figure 9 shows a schematic diagram of the pressure wave generator with a pressure regulator.

[0068] Figure 10 shows a partial schematic view of the pressure wave generator with a pump and heat exchanger.

[0069] Figure 11 shows a diaphragm spacer with slots for cooling.

[0070] DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

[0071] According to various aspects of the present invention as illustrated in figures 1 -1 1 , there is provided a pressure wave generator 1 which will now be described. It will be appreciated that these figures illustrate the general principles of the structure and construction, and that the invention is not limited to the precise configurations illustrated.

[0072] The present invention relates to a pressure wave generator 1 , as shown in figure 1. The pressure wave generator 1 is configured to generate pressure waves that may be used for a range of different applications.

[0073] In some configurations, the generated pressure waves from the pressure wave generator 1 are used in a cryogenic system 1000. In these cryocooling applications, components of the pressure wave generator 1 operate such that gases in the system are caused to compress and expand. This in turn causes the temperature of the gas to increase or decrease. The cryogenic system 1000 uses the gas temperature decreases and generated pressure waves, to provide a cooling effect.

[0074] The pressure wave generator in some configurations is used for driving cryogenic refrigerator systems, such as, for example in Stirling cryocoolers and pulse tube cryocoolers.

[0075] Other than cryogenic systems, the pressure wave generator 1 may be used in a range of other systems for other applications requiring reciprocating pressure waves. For example, the pressure wave generator could be used as a driver for a steady flow compressor by the addition of non-return valves. In these configurations, the working fluid may be oxygen for example. Ports of the housing, such as first port 31 and second port 32 may be provided with non-return valves so that one port acts as an inlet and one port acts as an outlet.

[0076] As shown in figure 1 , there is a pressure wave generator 1. The pressure wave generator 1 comprises a housing 21. The housing 21 comprises a first port 31 and a second port 32 for inflowing and outflowing pressure waves generated within the pressure wave generator. It should be appreciated, the openings / ports identified by reference numerals 31, 32 may be either inlets or outlets depending on the direction of gas flow.

[0077] The housing 21 may generally have side walls 22 and end plates 23, 24. It will be appreciated that the housing 21 could be generally cylindrical, box-shaped or any other shape of housing as desired. The housing 21 may be metal, for example steel, for heat conduction and strength, or aluminium or other materials could alternatively be used.

[0078] The pressure wave generator 1 further comprises a multiple diaphragm assembly 41. The multiple diaphragm 41 is located within the housing 21 of the pressure wave generator. As shown in figure 1 , the multiple diaphragm 41 comprises a first diaphragm 45 and a second diaphragm 47. In other words, the multiple diaphragm is a set of diaphragms having a first diaphragm 45 and a second diaphragm 47. The first diaphragm 45 and the second diaphragm 47 are moveable in a reciprocating motion within the housing 21 to generate the pressure waves.

[0079] As used herein, the multiple diaphragm 41 is a diaphragm configuration comprising two or more diaphragms.

[0080] In the configurations shown, the multiple diaphragm 41 is a double diaphragm configuration having two diaphragms being the first diaphragm 45 and the second diaphragm 47. In alternative configurations, the multiple diaphragm 41 may have more than two diaphragms, such as three, four, or more diaphragms. It is anticipated that the pressure wave generator has at least two diaphragms, configured to operate in unison, to provide the advantages over existing systems, as described below.

[0081] As shown in the figures, a gap 110 is provided between the first diaphragm 45 and the second diaphragm 47 in the multiple diaphragm 41 . The gap 1 10 separates and spaces the first diaphragm 45 apart from the second diaphragm 47.

[0082] When more than two diaphragms are provided in the multiple diaphragm 41, gaps 1 10 may be provided between respective pairs of the diaphragms in the multiple diaphragm 41.

[0083] Despite the presence of a gap 110, the first and second diaphragm 45, 47 form a set i.e. the multiple diaphragm 41, as the first and second diaphragms are operatively located or connected with each other, at least because they are located adjacent to each other / are close together. In these configurations, the first and second diaphragms 45, 47 are configured to move together upon movement of the moving assembly 61 . This gap 1 10 between the first and second diaphragms 45, 47 may be a vacuum or fluid filled space, as later described.

[0084] When more than two diaphragms are provided in the multiple diaphragm 41 , each of the first, second, third, etc diaphragms are operatively located or connected with each other and are configured to move together upon movement of the moving assembly 61. The gaps 1 10 between respective pairs of the diaphragms in the multiple diaphragm may be a vacuum or fluid filled space, as later described.

[0085] The description herein relating to the first and second diaphragms 45, 47 and their related features and functionality can equally apply to the third or additional diaphragms in the multiple diaphragm 41.

[0086] As best shown in the diagram provided in figures 4 and 5, the first and second diaphragms 45, 47 have an inner attachment region 151 , 152 connected to a moving assembly 61 of the pressure wave generator 1 , and an outer edge 155, 156 connected to the housing 21.

[0087] In some configurations, as shown in the exemplary figures, the flexible portions of the diaphragms 45, 47 are annular. However, it will be appreciated that the flexible portions of the diaphragms 45, 47 need not necessarily be annular in alternative forms of the pressure wave generator 1 .

[0088] The outer peripheries of the diaphragms 45, 47 may have any suitable shape, depending on the shape of the housing 21. In one exemplary configuration, the outer peripheries of the diaphragms 45, 47 are circular.

[0089] In some configurations, the diaphragms 45, 47 are approximately 500mm to 1000mm in diameter. In some configurations, both diaphragms 45, 47 are approximately 500mm in diameter. In other configurations, both diaphragms 45, 47 are approximately 800mm in diameter.

[0090] In some configurations the outer edges 155,156 of the multiple diaphragm 41 are attached to the housing 21 toward a housing end plate 23, as indicated in figures 1 -3.

[0091] It should be understood that the outer edges 155, 156 are connected to the housing 21 , which typically fixed in place, while the inner attachment regions 151, 152 are displaced by the moving assembly 61 such that each diaphragm has a fixed edge, and a moving portion or edge.

[0092] The outer edges 155, 156 may be connected to the housing 21 directly as shown in figures 1 and 6.

[0093] In other configurations, outer edges 155, 156 may be connected to the housing 21 indirectly as shown in figures 2 and 5. In these examples, the outer edges 155, 156 may be connected to a housing spacer 81. The housing spacer 81 may be a ring located between an end plate 23, and a side wall 22 of the housing 21.

[0094] A benefit of having the housing spacer 81 is that it enables a diaphragm to be used that has a flat outer ring of the diaphragm clamped between the end plate 23 and the side wall 22 of the housing.

[0095] As shown, the first and second diaphragms 45, 47 may be separated by one or more spacer components. The spacer components may be provided with adherence or fastening features to which one or both diaphragms 45, 47 may be attached.

[0096] The inner attachment regions 151 , 152 of the first and second diaphragms 45, 47 may be securely fixed at or toward an end of the moving assembly 61 . Like the outer edges 155, 156, the inner attachment regions 151 , 152 may be connected to the housing 21 directly as shown in figure 1 , or indirectly as exemplified in figures 2-5. For example, the inner attachment region 151 , 152 may be connected to an inner spacer 95. The inner spacer 95 may be a plate located on top of the moving assembly 61 .

[0097] In some configurations, the pressure wave generator 1 has both a housing spacer 81 (located at the other periphery of the diaphragms) and the inner spacer 95 (located at an inner region of the diaphragms). The spacers, allow for the gap 1 10 to be formed between the first and second diaphragms 45, 47. The gap 1 10 between the diaphragm is designed to be sufficient for a fluid to be introduced and stored between the diaphragms, and to prevent the diaphragms from touching (i.e. rubbing each other which would wear down the diaphragms). The spacers 81 , 95 may have a thickness of 5mm to 25mm.

[0098] It will be appreciated that there are various ways of fixing the inner attachment regions 151 ,152 and outer edges 155,156 of the multiple diaphragm 41 to the moving assembly 61 and the housing 21 respectively. For example, the diaphragms 45, 47 may be coupled or fixed to the housing 21 and moving assembly 61 by fastening components, welding, brazing and / or adhesives. There may be apertures in the diaphragms through which fasteners can be inserted, or the diaphragm may be clamped in place by the gas pressure. This is possible, as in some configurations, one or both of the diaphragms 45, 47 may be circular in shape such that the diaphragms may be placed across a surface (end plate 62 or associated spacer 95) of the moving assembly 61 .

[0099] In some configurations, the inner attachment regions 151 ,152 of the multiple diaphragms 41 may not be actively secured to the moving assembly 61 , but in configurations may simply be placed on the surface of end plate 62 / spacer 95 of the moving assembly 61. The connection is provided as the diaphragms are clamped in place by the gas pressure created within the pressure wave generator 1 , or held / sandwiched in place between components of the pressure wave generator. The pressure differential between the interspace fluid in the gap 1 10, and the fluid in the compression space 101 allows for this, and will be described in detail below.

[0100] A drive system 51 is provided to drive the moving assembly 61. As the moving assembly 61 , is driven to move, the multiple diaphragm 41 moves in a reciprocating motion within the housing 21 to generate pressure waves. The reciprocating motion of the multiple diaphragm 41 is provided as the inner attachment regions 151,152 are attached, directly or indirectly, to the moving assembly 61 and motion from the moving assembly is transferred to the diaphragms.

[0101] The moving assembly 61 , can move between a first position shown in figure 2, and a second position shown in figure 3. Figure 3 shows the pressure wave generator having a downwardly displaced moving assembly 61. A compression space 101 provided between the end plate 23 and the moving assembly 61 and the multiple diaphragm 41 expands and compresses as the moving assembly moves. We note that these are schematic drawings, and the exact movement and the shown deformation and movement of the multiple diaphragm 41 is for the purposes of demonstrating the concept of movement and deformation and may not be a true representation of the changes during movement.

[0102] It will be appreciated that the moving assembly 61 of the pressure wave generator 1 may be formed in various ways. In one configuration, as shown in figure 1 , the moving assembly 61 is in the shape of a cylinder that has an outer wall 64 and end plates 62, 63. It will be appreciated that the multiple diaphragm 41 and end plates 62, 63 of the moving assembly 61 can act as heat exchangers to remove heat due to compression of gas. The side walls 22 of the housing 21 can be considered as being a tension frame that holds the end plates 23, 24 of the housing together against the total gas pressure generated by the pressure wave generator 1 .

[0103] Various drive systems 51 may be arranged to manipulate the diaphragms in the pressure wave generator 1 in a reciprocating motion. One example of a suitable drive system 51 is the reciprocating moving assembly 61 forming part of a piston assembly. The moving assembly 61 is driven back and forth in the directions shown by arrows A and B.

[0104] The moving assembly 61 is driven by an operable actuator, for example a connecting rod (conrod) 91 and crank shaft 92 arrangement, as shown in figure 1. One end of the conrod 91 is pivotally coupled to a connecting component 94 within the moving assembly 61 at a conrod coupling 93 and the other end of the conrod is operatively coupled to a crank of the crank shaft 92. As the crank shaft 92 rotates, the moving assembly 61 is driven in a reciprocating motion by conrod 91 and this in turn causes the multiple diaphragm 41 coupled to the moving assembly 61 to move back and forth in a reciprocating motion. It will be appreciated that various alternative drive systems may be utilised to manipulate the attached diaphragms in a reciprocating motion. As shown in figures 1 and 3, the multiple diaphragm 41 is arranged to form a compression space 101 within the housing 21. In the example shown in figure 1, the compression space 101 is formed by the multiple diaphragm 41 and the end plate 23 of the housing 21. The multiple diaphragm 41 is supported and driven by the moving assembly 61.

[0105] It will be appreciated that other configurations may alternatively be used.

[0106] In operation, the multiple diaphragm 41 moves in a reciprocating motion and the volume of the compression space 101 changes, to generate reciprocating pressure waves. The pressure waves generated pass or flow through the first or second port 31 , 32 provided in the end plates 23, 24 of the housing 21 . The pressure waves may be used to drive a system connected to the first or second port 31 , 32. For example, the pressure waves may be used to drive a cryogenic system 1000 (figure 7).

[0107] In the application of a cryogenic system, the pressure wave generator 1 isolates the operating gas of the cryogenic systems from harsh environment associated with the drive system. In particular, the compression space 101 is sealed from the moving actuator parts of the drive system 51 , for example the conrod 91 and crank shaft 92. This enables the conrod 91 and crankshaft 92 to be located in a well lubricated chamber 65 for long life, but also enables the operating gas in the compression spaces 101 to be free of contaminants, such as hydrocarbon lubricants, for efficient performance of the cryogenic system.

[0108] In some configurations, the compression space 101 may be provided with a volume of operating gas, such as helium, hydrogen or oxygen. In operation, the reciprocating multiple diaphragm 41 creates pressure waves of the operating gas for driving the cryogenic system 1000 via the inlet outlet port 31 , 32 of the housing 21.

[0109] In some configurations, with reference to figure 9, the interspace gas located in the gap 1 10, is the same type of gas as the gas in the compression space 101. In these configurations, the benefit of using the same type of gas in these regions allow for a simple arrangement and also reduce the potential contamination within the pressure wave generator 1. For example, if the first (upper) diaphragm 45 fails, the interspace gas from the gap 1 10 will leak and equalise with the pressure of the compression space 101 . It will be appreciated that a pressure wave generator 1 with multiple diaphragm 41 can be arranged to drive one or more cryogenic systems 1000 of varying types, or configured for use in other applications.

[0110] Now turning to figures 2 and 5, there is clearly shown the multiple diaphragm 41 with a gap 110 formed between the first diaphragm 45 and the second diaphragm 47. In some configurations, the gap 1 10 between the diaphragms may comprise a vacuum. In other configurations the gap 1 10 is filled with a fluid (the interspace fluid being the fluid between the first and second diaphragm). The interspace fluid is under pressure and may be a gas or a liquid.

[0111] In general, the gap 1 10, and thus the distance between the first and second diaphragms 45, 47, is sufficiently large such that the diaphragms do not rub together or fret when reciprocating or when under high pressures.

[0112] The spacing between the diaphragms 45, 47 will typically remain substantially constant as the multiple diaphragm 41 is moved by the moving assembly 61.

[0113] In some configurations, the first diaphragm 45 is spaced at least 5mm from the second diaphragm 47 (indicated by the arrow in figure 5). In some configurations, the first diaphragm 45 is spaced between 5mm to 25mm from the second diaphragm 47. The spacing between the first and second diaphragms 45, 47 is held at a minimum of 5mm apart, to prevent touching of the diaphragms and to allow for the gap to be filled with liquid or gas. This may be differentiated from systems where multiple diaphragms may be present, however the diaphragms are stacked and are touching each other. In these systems, there is no gap for retaining a liquid or gas, and therefore does not provide for the advantages as disclosed herein including leak detection capabilities, and preferential leak direction due to pressure differentials.

[0114] In one example configuration, the first diaphragm 45 is spaced approximately 10mm from the second diaphragm 47.

[0115] It should be appreciated that configurations having a multiple diaphragm 41 may have advantages such as increased lifetime of the diaphragm and thus lifetime and improve safety of the pressure wave generator 1 and system as a whole. In a pressure wave generator 1 with multiple diaphragm 41 , if one diaphragm is to fail, the other diaphragm(s) may provide redundancy and prevent a leak. The second diaphragm 47 is located close the first diaphragm 45 and may operate to provide similar sealing required. Further, longer diaphragm life is achievable because the mean pressure is shared between the presence of two diaphragms. Diaphragm life may be extended by altering the stresses experienced by the diaphragms, and the presence of a second diaphragm 47 can mitigate the average pressure experienced by the first diaphragm 45 and therefore fatigue and eventual failure due to stresses experienced. This second diaphragm 47 may see comparatively lower loads compared with the first diaphragm 45 of the multiple diaphragm 41. Pressure may therefore be contained by the second diaphragm 47 for some time after the failure of the primary / first diaphragm 45.

[0116] This safety feature (and potential leak detection capabilities of the system to be discussed later) means that use of hazardous gases may be possible in the system. This may be advantageous in some applications, for example where a cryocooler uses hydrogen as the working fluid, this can provide more cooling power (for example 10 to 27% more) in comparison to helium being used in the same cryocooler. However, it should be understood that the presently described pressure wave generator 1 having two diaphragms, can improve lifespan of the components and will provide advantages over known systems, regardless if a hazardous gas is the working fluid or not.

[0117] Not only do the additional diaphragm(s) provide a redundancy, the outer diaphragms, such as first and second diaphragms 45, 47, may work in synergy, as the loads experienced by the multiple diaphragm 41 can be spread across both diaphragms, such that each diaphragm may be subjected to lower loads this de-stressing the diaphragms in the system. This can increase the lifetime of the multiple diaphragm 41 or enable greater charge pressures for a higher output of the system being driven. For example, a multiple diaphragm arrangement may increase the lifespan of the system by more than double compared to an equivalent system using single diaphragms. In one example, a pressure wave generator having multiple diaphragms 41 as herein, may operate for 100,000+ hours, while an equivalent pressure wave generator with single diaphragms may only operate for 40,000 hours. The multiple diaphragm 41 pressure wave generators may also enable higher charge pressure for more pressure generated output in the connected system of the pressure regenerative. The multiple diaphragm 41 has the potential to de-stress each diaphragm to less than its endurance limit, meaning that the diaphragms cannot fail due to fatigue. In some configurations, the drive system 51 operates to move the moving assembly 61 such that the multiple diaphragm 41 is actuated at a high frequency (up to 60 Hz). In some configurations, the diaphragms 41 are actuated at a frequency of 30 Hz to 50Hz. The distance travelled by the diaphragms may be approximately 5mm (e.g. due to the material, construction and size of the diaphragm). Systems using a single diaphragm may not operate at these higher frequencies, or at least may not have as long a lifespan compared to the pressure wave generators having a multiple diaphragm 41 as herein described.

[0118] The pressure wave generator 1 may be able to operate at these higher frequencies, largely due to the presence of the multiple diaphragm 41. The diaphragms may by destressing (reducing or sharing of load between the first and second diaphragms to improve the lifespan of the diaphragms) by having two (or more) diaphragms, provide scope for the diaphragms to be less dished / less curved. In these configurations, the less dished diaphragms may allow for greatertravel (movement of the moving assembly) in the pressure generator, which in turn improves cooling power, and thus improves efficiency of the system attached (e.g. cryocooler).

[0119] Furthermore, the presence of the gap 1 10 between the first and second diaphragms 45, 47 may be tapped into for the additional purposes e.g. leak detection.

[0120] In some configurations, as shown in figure 2, and best shown in figure 5, the pressure wave generator 1 has a leak detection port 98 in fluid communication with the gap 1 10.

[0121] As the gap 1 10 may have a vacuum, or fluid-filled and under pressure, changes (e.g. in pressure) between the first and second diaphragms 45, 47 may be detected and responded to. The pressure wave generator 1 may have a sensor or other component 99 connected to the leak detection port 98 to detect or respond to changes in the vacuum or pressure of the gap 110.

[0122] In some configurations, the gap 110 between the first diaphragm 45 and second diaphragm 47 in the multiple diaphragm 41 is a vacuum. In these configurations, the vacuum force may hold the first diaphragm 45 and second diaphragm 47 in place by providing a force which acts on both diaphragms, pulling them together. The vacuum force may act to clamp the diaphragms onto the spacer components 95, 81. An advantage of having a vacuum filled space is that this can remove the need for fasteners to keep the diaphragms in place. Alternatively, the vacuum force may supplement the fastening or adherence mechanisms, or the fastening or adherence mechanisms may only be intended for redundancy in the case the vacuum fails.

[0123] In configurations wherein the gap 1 10 comprises a vacuum, the pressure wave generator may also comprise a vacuum switch 99 to respond to a failure of the multiple diaphragm 41 . The vacuum switch is tapped into the gap 1 10. When the vacuum is broken, for instance in the case of a leak in the gap 110 or if one or both of the diaphragms 45, 47 are damaged, the vacuum switch 99 moves from a first switch position to a second switch position. In some configurations, the vacuum switch 99 may trigger a shut-down of the system if a leak is detected i.e. the vacuum has been broken.

[0124] The vacuum configurations may be advantageous where there is merit in maintaining thermal insulation between the drive mechanism and the working fluid.

[0125] In other configurations, the gap 1 10 between the first diaphragm 45 and second diaphragm 47 may comprise a gas under pressure. The gas under pressure may be an inert gas e.g. helium or hydrogen.

[0126] The pressure of the fluid in the gap 1 10 between the diaphragms 45, 47 is controlled to provide a specific interspace pressure. The pressure of the fluid in the gap 1 10 is provided to be as high as possible, while being limited by the lowest pressure in the compression space 101. Too much or too little pressure exhibited by the fluid in the gap 1 10 could result in loss of performance and possible machine malfunction. The pressure of the pressurised fluid in the gap 1 10 is therefore provided to be only slightly lower than the minimum pressure in the compression space 1 10. In other words, the pressure of the pressurised fluid in the gap 110 is configured to be lower than the mean pressure minus the amplitude of the pressure (i.e. the lowest pressure) in the compression space. The reason is that if at any time the interspace pressure of the pressurised fluid is greater than the compression space 101 pressure, then the diaphragms can separate from the spacer(s) 81 , 95. The first and second diaphragms 45, 47 hold the spacer(s) in place by the pressure. A benefit of providing specific interspace pressure is to ensure the first and second diaphragms 45, 47 are held in place by the pressure differential, thus removing the need to weld or otherwise hold the diaphragm together via other means, and therefore simplifying the manufacturing process and reducing the likelihood of potential failure.

[0127] In some configurations, the compression space pressure 101 is approximately 25 bar plus or minus 5 bar. The interspace fluid pressure in the gap 1 10 is approximately 10 bar to 20 bar.

[0128] In some configurations, with reference to figure 9, a pressure regulator 1 12 may be connected to the gap 1 10 to introduce and / or maintain the interspace pressure. The interspace pressure may be about 50% to < 100% of the compressor space 101 or gas spring space 102. As shown in figure 1, a connection 120 is provided between the compression space 101 and the gas spring space 102 to ensure the pressures in the two spaces are approximately the same. In the configuration, shown in figure 9, the pressurised feed to the gap 1 10 is provided from the gas spring space 102 because the pressure variation in this space is small (due to a larger volume), compared to the pressure variation in the compression space 101 (to drive the cold head and to produce cooling). However, the pressure in the two regions are ultimately the same, or similar. As described herein, having the interspace pressure in the gap 1 10 being lower compared to the compression space 101 may be desirable, and by providing a feed from the gas spring space 102, and regulating the pressure so that the interspace pressure is lower than the gas spring space pressure effectively achieves this pressure profile. The gas spring space 102 is the space between the end plate 63, and the bottom of the housing 24.

[0129] As shown in figure 9, in one configuration, the pressure regulator 1 12 receives a pressurised fluid from a source, e.g. the gas spring space 102, and introduces that fluid (at a slightly lower pressure) to the gap 1 10 between the diaphragms 45, 47. This feed of fluid allows for a controlled feed of pressurised gas to the gap 1 10 between the diaphragms.

[0130] Optionally, the feed circuit for the interspace gap includes a pressure relief valve 1 13 to ensure that the double diaphragm interspace pressure is lower than the gas spring space pressure 102. The pressure relief valve 113 may vent gas (to atmosphere) to reduce the pressure of the fluid fed to the gap 1 10 and / or to maintain the lower pressure in the gap compared to the gas spring space pressure 102. A pressure switch 114 may be provided to open or close the circuit depending on the pressure of the fluid, i.e. if the pressure of the fluid is higherthan desired the circuit may open.

[0131] Further, a pressure sensor 99 may be provided to determine the pressure of the fluid being fed or received from the gap 110, e.g. before allowing for venting to reduce pressure if required. The pressure sensor may detect an increase in pressure which is an indication of leakage from the gap 1 10 between the double diaphragms 45, 47 or a reduction in pressure may confirm that venting has occurred successfully.

[0132] Alternatively, a check valve 116 may be provided to ensure the double diaphragm interspace pressure is not significantly higher than the gas spring space pressure 102 and allows flow in the direction away from the gap 1 10, and towards gas spring space 102. In these configurations, where a check valve 116 is provided, a pressure relief valve may not be necessary to reduce pressure.

[0133] In other configurations, other high-pressure sources in the pressure wave generator system, or external to the system may be used. For example, buffer tanks that are part of the tuned system to promote resonance in a pulse tube circuit may be used to pressurise the gap 1 10.

[0134] The double diaphragm 45, 47 and specific pressurised fluid and specific interspace pressure profile can be contrasted to other systems, where there may be no gap 1 10 and therefore no pressurised fluid present. In those systems where there is no fluid present, the pressure in the gap if one exists between diaphragms is atmospheric (i.e. not pressurised as provided in the present in the pressure wave generator herein described). These systems do not provide the advantages of the specific assembly of the diaphragms 45, 47 as described above, nor the potential leak detection capabilities which rely on detecting pressure changes.

[0135] In other configurations, the gap 1 10 between the first diaphragm 45 and second diaphragm 47 may comprise a liquid. The gap 110 may be completely filled with the liquid, such that there are no gaseous bubbles. The liquid can act as a coolant to assist with dissipating heat resulting from the compression of gas. Further, the liquid being an incompressible fluid allows for equal load to be distributed between the two diaphragms reducing stresses experienced by any one diaphragm, and assists the diaphragms to move in unison. The liquid can be any suitable liquid, and in one exemplary configuration may be oil.

[0136] The liquid may assist with cooling by circulating within the gap 1 10 between the diaphragms. In some configurations, a liquid may be circulated into and out of the gap 1 10 by a pump 130, as shown in figure 10. The pump 130 may be an oil, water or other liquid pump. A heat exchanger 131 may also be provided to cool the fluid in the gap 110. In this configuration, the liquid may pass through a cooling system when outside the gap 1 10. This may provide even more effective heat dissipation. Alternatively, the liquid may simply be moved around within the gap 1 10 as a result of the motion created by the reciprocation of the diaphragms and moving assembly.

[0137] With reference to figure 1 1 , the inner spacer 95 between diaphragm(s) 45, 47 may include fine slots on a surface to enable cooling flow overthe diaphragm face. Alternatively, a woven metal mesh may be provided or another porous spacer with sufficient compressive strength to maintain separation while enabling cooling.

[0138] In some configurations, having a fluid in the gap 1 10 between the first and second diaphragms 45, 47 may reduce peak stresses experienced by the pressure containing diaphragms. This may improve their lifespan of the diaphragms 45, 47 in the pressure wave generator 1.

[0139] There are two main factors influencing peak stresses on diaphragms (and hence tendency for premature failure): 1 ) the gas pressure that is contained, and 2) the deflection ortravel distance. Having a gap between the two diaphragms pressurised to an intermediate (interspace) pressure results in reduced pressure experienced by each diaphragm. In some configurations, the interspace gas is initially pressurised and then sealed between the diaphragms to provide the gas under pressure in the gap 1 10. Because this reduces the stress due to pressure containment, there is scope to design for greater diaphragm travel. Further, the shared and reduced stresses experienced by each diaphragm 45, 47 allows the pressure wave generator 1 to operate at relatively high frequencies e.g. up to 50Hz or 60Hz, with reduced risk of failure.

[0140] In some configurations, one of the two diaphragms 45, 47 may be designed to fail first, to control diaphragm failure. In one configuration, the second (lower) diaphragm 47 is provided with a weakened region or weakened structure (e.g. a thinner material, weaker material composition, mechanical distressing to cause wear) to ensure the second diaphragm fails before the first (upper) diaphragm 45. This may be beneficial if the interspace fluid is different from the compression space 101 working gas. The interspace fluid may be the same as fluid used in driving system e.g. hydraulic oil. The hydraulic oil may be compatible with the pressure wave generator 1 lubricating fluid, and thus there is no contamination in the event of the second diaphragm 47 failure.

[0141] In other configurations, the first diaphragm 45 is provided with a weakened region or weakened structure to ensure the first diaphragm fails 45 before the second diaphragm 47. These may be useful in configurations, where the interspace fluid in the gap 1 10 is the same as the working gas in the compression space 101.

[0142] In some configurations, leak detection is provided where a parameter of the gap 1 10 of the multiple diaphragm is measured / detected, and a change in the parameter provides an indication of a leak of the multiple diaphragm 41. For example, detecting a change in pressure of the gap 110 below or above a threshold pressure may be an indication of a leak of the multiple diaphragm 41.

[0143] In some configurations, the pressure in the gap 110 (when the diaphragms have not failed) is greater than the pressure of other spaces within the pressure wave generator 1 . For example, the pressure in the gap 110 is greater than either or both the compression space 101 (the space between the end plate 23, 24 of the housing 21 ), a housing chamber 71 (the space between the housing side walls 22, and the moving assembly side wall 64). Typically, the expected pressure in the gap 1 10 is greater than each of the compression space 101, and pressure housing chamber 71.

[0144] A reduction of pressure from the expected pressure of the gap 110 may be an indication of failure of the multiple diaphragm 41.

[0145] In some configurations, the pressure wave generator 1 may also have a pressure sensor 99 fluidly connected to the gap 110.

[0146] The pressure sensor 99 may be located in any suitable position. In one exemplary configuration, the pressure sensor 99 may be fitted to the pressure wave generator through a port in the outermost spacer between the diaphragms.

[0147] In some configurations, the expected pressure of the gap 110 is 5 to 15 bar. In some configurations, the pressure wave generator 1 may be configured such that readings from the pressure sensor 99 may be used to trigger an action such as shutting off power to drive system 51. If the pressure sensor 99 detects a reading lower than or higher than an expected threshold pressure, this may be an indication of a leak.

[0148] In other configurations, the pressure wave generator 1 may be configured such the pressure sensor 99 detects the rate of pressure change. In these configurations, the pressure sensor 99 may be used to determine (and provide advance notice) of when maintenance could be carried out for the pressure wave generator 1 or if the multiple diaphragm 41 needs replacing.

[0149] For example, a slow rate of decrease in pressure may indicate that the diaphragms 45, 47 require maintenance, but that there is not an immediate risk of failure. It should be appreciated that having an indication of the rate of change of pressure means that advance notice before total failure is provided. This is advantageous feature is not possible with pressure wave generators where there is only one diaphragm. Even where pressure wave generators may have two diaphragms in the system, they may not be operatively located (i.e. there is no de-stressing ability as they are not close enough to share the load), and / or the pressure between the diaphragms are not monitored by a pressure sensor for example for advance leak detection purposes.

[0150] In some configurations, the pressure wave generator 1 may be configured to determine which of the two diaphragms 45, 47 has failed based on the readings from the pressure sensor. If the nominal charge pressure in the gap 1 10 is between the pressure in the housing chamber 71 and the lowest pressure in the compression space during the compression cycle, then a decrease in the gap 1 10 pressure would indicate a failure of the diaphragm adjacent to the housing chamber 71 (shown as the second diaphragm 47), and an increase in the gap 1 10 pressure would indicate a failure of the diaphragm adjacent (shown as the first diaphragm 45) to the compression space 101 .

[0151] In some configurations, there may be two pressure sensors. In some applications, the additional sensor is included as a secondary pressure sensor so that if the first pressure sensor fails or is faulty, there is a back-up sensor. In other configurations the readings of both pressure sensors are averaged to determine the pressure of fluid within the gap 1 10. In further configurations, there may be more than two pressure sensors. In some configurations, the pressure wave generator 1 can detect if one or both of the diaphragms 45, 47 in the multiple diaphragm 41 has failed or is defective, and additionally optionally, which one has failed or is defective.

[0152] In some configurations, the pressure wave generator 1 is part of a system 1000 having a controller. The controller may receive an input from the sensor 99, such that when a leak is detected (e.g. a pressure drop or high rate of pressure change), the controller communicates with one or more components of the system in response. For example, the controller may communicate with the drive system 51 and shut off power in response to detecting a leak. In addition, or alternatively, when the controller receives a reading from the sensor 99 outside the normal expected operating range, the controller may initiate a warning (visual and / or audible) to indicate there is a leak or maintenance may be due soon.

[0153] In some configurations, the warning that is initiated by the controller may be a remote warning notified via an internet, wireless, or cloud-based system.

[0154] If one of the diaphragms 45, 47 has failed, the system may shut down automatically i.e. a shut-down protocol may be triggered. Alternatively, the system may provide a warning that a shut-down is necessary. In some configurations, the system may provide a warning, and this warning may be used to aid in determining when maintenance should be carried out to assess the condition of the diaphragms 45, 47 or other aspects of the system.

[0155] In some configurations, the pressure wave generator system 1 may be configured to shut down based on readings from the pressure sensor or the vacuum switch detecting a broken vacuum. A change in the pressure within the gap 110 or the vacuum switch switching can be an indication that there has been a diaphragm failure. Shutting down the system in these circumstances can prevent hazardous gas leaking into the piston chamber 71 of the pressure wave generator 1 or out of the pressure wave generator.

[0156] In some configurations, the pressure wave generator system may be configured to shut down when the pressure within the gap 110 drops below a certain threshold. This threshold may be between 2 bar and 8 bar. In other configurations, the pressure wave generator system may be configured to shut down when the pressure deviates from a baseline pressure by a certain amount, for example when the pressure decreases from a baseline pressure by 25%. As such, the pressure sensor 99 should be configured to be capable of taking pressure readings at regular increments, example at 60 second intervals. As described above, it should be appreciated that in these configurations where the multiple diaphragm 41 may be used for leak detection purposes, advantages include the ability to monitor and provide regular maintenance (e.g. to replace the diaphragms) before complete failure of the system. In some configurations, the leak detection capabilities of the system also means that the working gas used in system may be hazardous (e.g. hydrogen may be used for improved cooling). This is possible as there are redundancies built into the system, to reduce the likelihood of safety concerns arising from the use of hazardous fluids.

[0157] As described above, the pressure wave generator 1 comprises at least one multiple diaphragm 41 . In some configurations, the pressure wave generator 1 may have more than one multiple diaphragm 41 that also operates the same or similar manner described above.

[0158] As shown in figure 1, the pressure wave generator 1 may have a pair of multiple diaphragms 41, 42. The pressure wave generator 1 may have a first multiple diaphragm 41 (with at least first and second diaphragms 45, 47) and a second multiple diaphragm 42 (with at least first and second diaphragms 46, 48). The first multiple diaphragm 41 may be located at or towards a first end of the moving assembly 61 , and the second multiple diaphragm 42 may be located at or towards a second end of the moving assembly 61 . For example, the first multiple diaphragm 41 may be located at or towards a top / first housing end plate 23 and the second multiple diaphragm 42 may be located at or towards a bottom / second housing end plate 24.

[0159] The first and second diaphragms of each multiple diaphragm are operatively located adjacent each other e.g. diaphragms 45 and 47 are located adjacent / close to each other; and diaphragms 46 and 48 are located adjacent / close to each other. Within each set of the multiple diaphragms, the reciprocating waves may substantially affect each other.

[0160] In contrast, the first multiple diaphragm 41 and the second multiple diaphragm 42 are not operatively located, in that the multiple diaphragms are spaced sufficiently apart that the reciprocating waves of one set of multiple diaphragms are not substantially affected by the other set of multiple diaphragms. In some configurations, the first multiple diaphragm 41 is spaced 400 mm to 600 mm apart from the second multiple diaphragm 42. In some configurations, the space is determined by the height of the moving assembly 61 , as the first and second multiple diaphragms 41 , 42 may be connected to the top and bottom end plates 62, 63 of the moving assembly 61 respectively.

[0161] In some configurations, the first multiple diaphragm 41 is located towards a top of the pressure wave generator 1 , and the second multiple diaphragm 42 is located towards a bottom of the pressure wave generator.

[0162] In these configurations, the pressure waves generated in compression space 101 may be 180 degrees out of phase with those generated in compression space 102.

[0163] The multiple diaphragms 41, 42 may be curved such that a pressure containing portion of the diaphragm is either concave or convex with respect to the gap 110 between the diaphragms. In some configurations, the multiple diaphragms 41 may both be curved inwards concavely towards the gap 1 10 (e.g. for where a vacuum is present).

[0164] In other configurations, the diaphragms may both be curved outwards convexly away from the gap 1 10.

[0165] In yet another configuration, one diaphragm may be curved inwards towards the gap 1 10 and the other diaphragm curved outwards away from the gap 1 10.

[0166] In yet another configuration one or both of the diaphragms may be substantially flat.

[0167] In some configurations, the diaphragms are curved such that the distance between them across the gap 1 10 from the housing to the moving assembly remains relatively constant. In other configurations, the radius of curvature of the first diaphragm 45 may be substantially different to that of the second diaphragm 47, so that the distance between the diaphragms across the gap 110 from the housing 21 to the moving assembly 61 varies.

[0168] The diaphragms may be made from metal (such as stainless steel) or any suitable flexible material such as, for example, rubber, polytetrafluoroethylene, or the like. The multiple diaphragms 41 may be formed from a material that can seal in the operating gas, for example helium, that drives a cryogenic system 1000 connected to the pressure wave generator 1.

[0169] As previously mentioned, the pressure wave generator 1 may be used in a cryogenic system 1000. In some configurations, a cryogenic refrigerator is in fluid communication and operatively connected to the pressure wave generator 1. In these configurations, the cryogenic refrigerator is driven by the pressure waves generated by the pressure wave generator 1 .

[0170] Figure 7 shows a schematic of an example cryogenic system 1000. In this example there is shown a pulse tube cryocooler 200. The cryogenic refrigerator may have a compressor, heat exchanger, regenerator, and a cold end.

[0171] In some configurations, the cryogenic refrigerator has more than one heat exchanger.

[0172] The cryocooler 200 is connected to the inlet / outlet 31 , 32 of the pressure wave generator 1 such that generated pressure waves may drive the components coupled at the inlet / outlet.

[0173] In some configurations, the pressure wave generator 1 is used for the basis of a hydrogen compressor. In some configurations, where leak detection capabilities are provided by the features described above, the system can sense diaphragm failure, and may trigger shutting down of the system before the hazardous gas can leak into unintended regions of the pressure wave generator. Being able to detect a potential diaphragm failure, and leak in advance may improve the safety of the system, as this reduces the likelihood of complete system failure and the potential of an explosion incident.

[0174] It should be appreciated that the pressure wave generator 1 may be used with other cryocoolers (e.g. Sterling cryocoolers), or in other applications not illustrated.

[0175] To those skilled in the art to which the invention relates, many changes in construction and widely differing embodiments and applications of the invention will suggest themselves without departing from the scope of the invention as defined in the appended claims.

[0176] This invention may also be said broadly to consist in the parts, elements and features referred to or indicated in the specification of the application, individually or collectively, and any or all combinations of any two or more of said parts, elements or features, and where specific integers are mentioned herein which have known equivalents in the art to which this invention relates, such known equivalents are deemed to be incorporated herein as if individually set forth.

Claims

- -CLAIMS1 . A pressure wave generator comprising: a housing comprising an inlet and an outlet for inflowing and outflowing pressure waves generated within the pressure wave generator; a multiple diaphragm comprising: a first diaphragm; and a second diaphragm; the first diaphragm and the second diaphragm each having an inner attachment region indirectly or directly connected to a moving assembly and an outer edge indirectly or directly connected to the housing; a gap provided between the first diaphragm and the second diaphragm, the first diaphragm is spaced at least 5mm from the second diaphragm to form the gap, and the gap comprise an interspace fluid being a gas under pressure or a liquid and; a leak detection port in fluid communication with the gap; and a drive system for driving the moving assembly, the first diaphragm and the second diaphragm moveable in a reciprocating motion within the housing to generate the pressure waves as the moving assembly moves.

2. The pressure wave generator as claimed in the previous claim wherein the first diaphragm is operatively located adjacent to the second diaphragm such that the first diaphragm and the second diaphragm are configured to move together upon movement of the moving assembly.

3. The pressure wave generator as claimed in any one of previous claims wherein the first diaphragm is spaced 5mm to 25mm from the second diaphragm.

4. The pressure wave generator as claimed in any one of the previous claims wherein the first diaphragm and second diaphragm are connected to the housing and moving assembly by fastening components, adhesives, welding, brazing and / or clamped in place by the pressure created within the pressure wave generator.

5. The pressure wave generator as claimed in any one of the previous claims wherein the first and second the diaphragms comprise outer edges connected to a housing spacer.

6. The pressure wave generator as claimed in any one of the previous claims wherein the first and second the diaphragms comprise inner attachment regions connected to an inner spacer.

7. The pressure wave generator as claimed in any one of the previous claims wherein a compression space is formed within the housing, and is formed by the multiple diaphragm and the end plate of the housing.

8. The pressure wave generator as claimed in the previous claim wherein the interspace fluid in the gap is the same as a working fluid provided in the compression space.

9. The pressure wave generator as claimed in any one of claims 7 or 8 wherein the interspace fluid has an interspace pressure being lower than the lowest pressure of the working fluid in the compression space.

10. The pressure wave generator as claimed in the previous claim wherein the interspace pressure in the gap is 50% to <100% of the pressure of the working fluid in the compressor space.1 1 . The pressure wave generator as claimed in the previous claim wherein the interspace pressure in the gap is 10 bar to 20 bar and the pressure of the working fluid in the compressor space is 25 bar plus or minus 5 bar.

12. The pressure wave generator as claimed in any one of claims 7 to 11 wherein a connection is provided between the compression space and a gas spring space located between the housing and an end plate, to ensure the pressures in the compression space and gas spring space are approximately the same.

13. The pressure wave generator as claimed in the previous claim wherein a feed is provided from the gas spring space to the gap to provide the interspace fluid.

14. The pressure wave generator as claimed in any one of the previous claims wherein a pressure regulator is connected to the gap to introduce and / or maintain interspace pressure.

15. The pressure wave generator as claimed in any one of the previous claims wherein a sensor or other component is connected to the leak detection port to detect or respond to changes in the pressure of the gap.

16. The pressure wave generator as claimed in any one of the previous claims wherein the gap is filled with an inert gas, optionally the inert gas is helium.

17. The pressure wave generator as claimed in any one of the previous claims, wherein the liquid in the gap is a coolant.

18. The pressure wave generator as claimed in the previous claim, further comprising a pump to pump the coolant through the gap.

19. The pressure wave generator as claimed in any one of the previous claims further comprising a pressure sensor fluidly connected to the gap.

20. The pressure wave generator as claimed in the previous claim wherein the pressure sensor detects the magnitude of pressure change in the gap.

21. The pressure wave generator as claimed in the previous claim wherein the pressure sensor detects the rate of pressure change in the gap.

22. The pressure wave generator as claimed in any one of the previous claims wherein the drive system is configured to operate the moving assembly such that the diaphragms is actuated at a frequency of approximately 30Hz to 50Hz.

23. The pressure wave generator as claimed in any one of the previous claims wherein the diaphragms are configured to travel approximately 5mm due to the reciprocating motion.

24. The pressure wave generator as claimed in any one of the previous claims wherein the diaphragms are 500mm or 800mm in diameter.

25. The pressure wave generator as claimed in any one of the previous claims wherein the second diaphragm comprises a weakened region or a weakened structure such that the second diaphragm is configured to fail before the first diaphragm.

26. The pressure wave generator as claimed in any one of previous claims 1 to 25 comprising a pair of multiple diaphragms, the pair of multiple diaphragms comprising a first multiple diaphragm and a second multiple diaphragm and wherein each of the first and second multiple diaphragms are multiple diaphragms as claimed in any one of the previous claims 1 to 25.

27. A method of detecting a leak in a multiple diaphragm pressure wave generator comprising: providing a pressure wave generator comprising a multiple diaphragm as claimed in any one of the previous claims; detecting a parameter in the gap of the multiple diaphragm; wherein a change in the parameter provides an indication of a leak of the multiple diaphragm.

28. The method of claim 27 wherein detecting a change in pressure in the gap below or above a threshold is an indication of a leak.

29. The method of the previous claim, wherein detecting a change in pressure below a threshold is an indication of a leak.

30. The method of any one of previous claims 27 to 29 wherein a pressure sensor is provided to detect the change in the parameter.

31. The method of any one of previous claims 27 to 29 further comprising detecting the rate of pressure change in the gap.

32. The method of the previous claim, wherein a first rate of pressure change indicates a failure of the first diaphragm, and a second rate of pressure change indicates a failure of the second diaphragm.

33. The method of any one of previous claims 27 to 32 further comprising triggering a shut down protocol when a leak has been detected.

34. A cryogenic system comprising: the pressure wave generator as claimed in any one of claims 1 to 25; and a cryogenic refrigerator operatively connected to the pressure wave generator such that the cryogenic refrigerator is driven by the pressure waves generated by the pressure wave generator.

35. The cryogenic system as claimed in the previous claim wherein the cryogenic refrigerator comprises a compressor, a heat exchanger, a regenerator, and a cold end.

36. The cryogenic system as claimed in claim 34 or 35 wherein the cryogenic system is a pulse tube cryocooler.

37. The cryogenic system as claimed in claim 34 or 35 wherein the cryogenic system is a Stirling cryocooler.

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